ar command does not produce index when combining static libraries - c++

Question

I am trying to follow the answer given here to combine multiple static libraries into a single archive under MinGW64. Specifically, I use CMake and specify the following command:
add_custom_command(
OUTPUT ${COMBINED_CORE_LIB_NAME}
COMMAND ${AR} -crs ${COMBINED_CORE_LIB_NAME} ${CORE_LIB_TARGET_FILES}
DEPENDS ${DILIGENT_CORE_INSTALL_LIBS_LIST}
COMMENT "Combining core libraries..."
)
Also, following the recommendation from here, I do not use stock ar, but rather cross ar:
find_program(AR NAMES x86_64-w64-mingw32-gcc-ar)
However, no matter what I do, ar refuses to generate index and every time I am trying to link against the produced library, I get this error:
error adding symbols: Archive has no index; run ranlib to add one
Running ranlib as suggested either stock one or x86_64-w64-mingw32-gcc-ranlib makes no difference.
I spent 15 minutes to make this work with MSVC and lib.exe and have been struggling for 8 hours with MinGW. Any suggestion would be highly appreciated.
[Edit]: It turned out that this problem is not really specific to MinGW and also happens on Linux in a very similar fashion.

Answer 1

As it turned out, it is not possible to merge multiple .a files into a single .a library using ar utility: it refuses to generate index. A working solution is to unpack all object files from all static libraries and then repack them into a combined library.
Below is the CMake instructions I ended up with:
set(COMBINED_CORE_LIB_NAME "${CMAKE_STATIC_LIBRARY_PREFIX}DiligentCore${CMAKE_STATIC_LIBRARY_SUFFIX}")
foreach(CORE_LIB ${DILIGENT_CORE_INSTALL_LIBS_LIST})
list(APPEND CORE_LIB_TARGET_FILES $<TARGET_FILE:${CORE_LIB}>)
endforeach(CORE_LIB)
if(MSVC)
add_custom_command(
OUTPUT ${COMBINED_CORE_LIB_NAME}
COMMAND ${LIB_EXE} /OUT:${COMBINED_CORE_LIB_NAME} ${CORE_LIB_TARGET_FILES}
DEPENDS ${DILIGENT_CORE_INSTALL_LIBS_LIST}
COMMENT "Combining core libraries..."
)
add_custom_target(DiligentCore-static ALL DEPENDS ${COMBINED_CORE_LIB_NAME})
else()
if(PLATFORM_WIN32)
# do NOT use stock ar on MinGW
find_program(AR NAMES x86_64-w64-mingw32-gcc-ar)
else()
set(AR ${CMAKE_AR})
endif()
if(AR)
add_custom_command(
OUTPUT ${COMBINED_CORE_LIB_NAME}
# Delete all object files from current directory
COMMAND ${CMAKE_COMMAND} -E remove "*${CMAKE_C_OUTPUT_EXTENSION}"
DEPENDS ${DILIGENT_CORE_INSTALL_LIBS_LIST}
COMMENT "Combining core libraries..."
)
# Unpack all object files from all targets to current directory
foreach(CORE_LIB_TARGET ${CORE_LIB_TARGET_FILES})
add_custom_command(
OUTPUT ${COMBINED_CORE_LIB_NAME}
COMMAND ${AR} -x ${CORE_LIB_TARGET}
APPEND
)
endforeach()
# Pack object files to a combined library and delete them
add_custom_command(
OUTPUT ${COMBINED_CORE_LIB_NAME}
COMMAND ${AR} -crs ${COMBINED_CORE_LIB_NAME} "*${CMAKE_C_OUTPUT_EXTENSION}"
COMMAND ${CMAKE_COMMAND} -E remove "*${CMAKE_C_OUTPUT_EXTENSION}"
APPEND
)
add_custom_target(DiligentCore-static ALL DEPENDS ${COMBINED_CORE_LIB_NAME})
else()
message("ar command is not found")
endif()
endif()
if(TARGET DiligentCore-static)
install(FILES "${CMAKE_CURRENT_BINARY_DIR}/${COMBINED_CORE_LIB_NAME}"
DESTINATION "${DILIGENT_CORE_INSTALL_DIR}/lib/"
)
set_target_properties(DiligentCore-static PROPERTIES
FOLDER Core
)
else()
message("Unable to find librarian tool. Combined DiligentCore static library will not be produced.")
endif()